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Buyers Guide to Digital Camera Sensor Technology

Buyers Guide to Digital Camera Sensor Technology - We explain the sensor technology used in todays digital cameras.

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Category : Digital Cameras
Product : PEN Mini E-PM1
Price : £286
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Sensor Technology

Complete guide to sensor technology, types, and what it means to image quality...


Sensor Technology

The imaging sensor sits in the middle of your digital camera, right behind the lens, and turns the optical image into a digital version, the quality, size and make up of the image sensor (combined with the lens and image processor) play a massive part in the image quality produced by the camera.

Olympus PEN Mini E-PM1
Olympus PEN Mini E-PM1

The sensor is made up of millions of pixels, and is made out of silicon wafer like computer chips, which you will have seen if you remember the Intel bunnymen adverts, a picture is shown below:

Kodak CMOS Wafer
From Kodak - Where they explain development of a CMOS sensor.
 
Each pixel point on a sensor is made up of a layer starting with:
  • On-chip microlens
  • Colour filter (bayer sensor or other)
  • Sensor aperture
  • Sensor area


From Sony PDF: Super HAD CCD II Diagram

Sensor types explained:


CCD - Charge-Coupled Device

Each pixel is responsible for collecting light, but as the pixel is simply detecting light, a colour filter is needed in order for the sensor to pick up the different colours in the scene. The Bayer Sensor pattern is the most popular and common arrangement of the filter, with a Blue, Green, Green, and Red arrangement:

Bayer Sensor Pattern
From Kodak.

CMOS - Complementary Metal–Oxide–Semiconductor. CMOS sensors use less power than CCD sensors and often allow quicker read speeds than CCD sensors, allowing high speed continuous shooting and high speed video.

Exmor CMOS sensor (Sony's CMOS sensors)
Exmor R CMOS sensor (Sony's name for backlit CMOS sensor, usually 1/2.33 inch or smaller)

Back-lit CMOS (also known as B.S.I / Back Side Illuminated) - Sony call it Exmor R Backlit CMOS - of benefit to small sensors such as camera phones, or compact cameras where the small size of the sensor means the sensor is mostly dominated by the wiring which blocks light from reaching the sensor, diagram from Sony's website:

Sony Backlit CMOS Sensor

And if you've ever wondered why it's called "Back-illuminated" when by the looks of things, they've actually moved the light detecting surface to the front / top of the sensor, it's because the light recieving surface is on the back side of the silicon layer. The benefits of back-lit CMOS sensors compared to front illuminated is shown below in this low light, 30lux, image from Sony:

Sony CMOS Backlit CMOS
Fujifilm EXR Sensor CCD/CMOS
First introduced in "Super CCD" form, Fujifilm used a diagonal "honeycomb" arrangement of pixels in the typical Bayer pattern, they then re-arranged the pixels and put blocks of colour pixels next to each other, creating the Super CCD EXR sensor:

Super CCD EXR Sensor Diagram

The EXR arrangement gives:
HR - High resolution - uses all pixels
DR - Dynamic range - combines neighbouring pixels reading different exposures
SN - Signal/noise sensitivity - combines neighbouring pixels

The EXR sensor is different from the Bayer sensor pattern, and Fujifilm has re-arranged the Red, Green, and Blue pixels to enable pixel combining: Super CCD EXR explained (PDF).

Fujifilm has also introduced a backlit CMOS version of the sensor, moving the wiring to increase sensitivity to light:


Image from Fujifilm (Whether EXR or Bayer pattern, the backlit technology is the same).

Another variation is the Fujifilm X-Trans CMOS sensor used in the Fujifilm X-Pro1 - this uses a new arrangement of the colour filter on the sensor designed to mimic film, and is explained in our Fujifilm X-Pro1 review.

Live MOS Sensor - Olympus and Panasonic use this name for their sensors, but there is little information on-line about the sensor technology. When used in compact cameras, it enables high speed shooting, much like backlit CMOS sensors. It is also used in Panasonic and Olympus Micro Four Thirds cameras such as the Panasonic Lumix G3.

Foveon X3 CMOS Sensor - Departing from the standard Bayer sensor, the Foveon sensor is unique in that it has the Red, Green, and Blue pixels on top of each other, so that at each point the camera is reading the full colour. The benefits of this seem clear, however Foveon sensors are only used in Sigma cameras, which means that development of the sensor and technology seems to be at a slower pace compared to the speed of development of other sensors, for example sensors in mobile phones are sold in their billions every year.


Image from Foveon

Side note: Sony HAD sensors use a variation of the bayer sensor pattern, and instead of having a square of red, green, blue, and green, they use red, green, blue and turquoise. More recently, Sony has again altered the colour filter and some of the latest sensors collect Red, Green, Blue and White for increased light sensitivity. More information here.

These are the different types of sensors available including the various technologies involved, whether one is better than the other is regularly debated, with the lens and image processing playing a bit part in the final image quality, which also includes levels of in camera JPEG compression or RAW processing. We'd also recommend reading our article on sensor size

We have seen the switch to backlit CMOS sensors improve image quality, with lower noise at higher ISO settings, higher continuous shooting speeds, high speed video and other benefits such as quicker AF.

Further reading: Sensor Size Explained

Side note: Some camcorders use "3CCD" sensors - this is literally three different sensors, and in front is a prism that splits the light to red, green, and blue and sends each colour to an individual sensor.

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